In cephalometry, the skull of a patient is X-rayed on a film from the side. Visible in a conventional X-ray image is then primarily the bone structure of a skull. It is often desirable, however, that the soft facial tissues of a patient would also be visible to a certain degree in a patient's skull. For this purpose, it is known in the art to adapt between the patient's skull and the film a V-shaped X-ray absorbing filter which prevents the X-radiation, which is penetrated through soft tissues and is in abundance with respect to the bony parts of a skull, from exposing the film too much on these areas as otherwise said soft tissues are not at all visible on the film. Since the shape and size of the patient's skulls differ considerably from each other, the position of said filter must be set patientwise in order to obtain each time an image exposed as properly as possible also for soft tissues. In practice, the positioning and setting of a filter is done by the operator according to his own judgement with the consequence that up to 2/3 of the images may be failures because of wrong filter position, in which case a patient may have to be re-exposed to X-radiation.